X25 - C H A P TER 17 X.25 Background X.25 is an...

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CHAPTER X.25 17-1 17 X.25 Background X.25 is an International Telecommunication Union–Telecommunication Standardization Sector (ITU-T) protocol standard for WAN communications that defines how connections between user devices and network devices are established and maintained. X.25 is designed to operate effectively regardless of the type of systems connected to the network. It is typically used in the packet-switched networks (PSNs) of common carriers, such as the telephone companies. Subscribers are charged based on their use of the network. The development of the X.25 standard was initiated by the common carriers in the 1970s. At that time, there was a need for WAN protocols capable of providing connectivity across public data networks (PDNs). X.25 is now administered as an international standard by the ITU-T. This chapter covers the basic functions and components of X.25. X.25 Devices and Protocol Operation X.25 network devices fall into three general categories: data terminal equipment (DTE), data circuit-terminating equipment (DCE), and packet switching exchange (PSE). Data terminal equipment devices are end systems that communicate across the X.25 network. They are usually terminals, personal computers, or network hosts, and are located on the premises of individual subscribers. DCE devices are communications devices, such as modems and packet switches, that provide the interface between DTE devices and a PSE and are generally located in the carrier’s facilities. PSEs are switches that compose the bulk of the carrier’s network. They transfer data from one DTE device to another through the X.25 PSN. Figure 17-1 illustrates the relationships between the three types of X.25 network devices.
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X.25 Devices and Protocol Operation Internetworking Technology Overview, June 1999 17-2 Figure 17-1 DTEs, DCEs, and PSEs make up an X.25 network. Packet Assembler/Disassembler (PAD) The packet assembler/disassembler (PAD) is a device commonly found in X.25 networks. PADs are used when a DTE device, such as a character-mode terminal, is too simple to implement the full X.25 functionality. The PAD is located between a DTE device and a DCE device, and it performs three primary functions: buffering, packet assembly, and packet disassembly. The PAD buffers data sent to or from the DTE device. It also assembles outgoing data into packets and forwards them to the DCE device. (This includes adding an X.25 header.) Finally, the PAD disassembles incoming packets before forwarding the data to the DTE. (This includes removing the X.25 header.) Figure 17-2 illustrates the basic operation of the PAD when receiving packets from the X.25 WAN. Figure 17-2 The PAD buffers, assembles, and disassembles data packets. Personal Computer Network Host DTE PSE X.25 WAN DCE Switch Modem 24117 Assembly/ Disassembly Buffer X.25 Data PAD 24118
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X.25 17-3 X.25 Session Establishment X.25 Session Establishment X.25 sessions are established when one DTE device contacts another to request a communication session. The DTE device that receives the request can either accept or refuse the connection. If the
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This note was uploaded on 08/08/2011 for the course CS 310 taught by Professor Aartisingh during the Spring '11 term at National Institute of Technology, Calicut.

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X25 - C H A P TER 17 X.25 Background X.25 is an...

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